Linear hydraulic actuators for opening or closing a gate valve are known. One known embodiment of a hydraulic actuator is used to operate a bottom gate valve in an inclined fuel transfer system of a particular type of boiling water reactor. The known hydraulic actuator has a pair of pin-actuated limit switches which indicate the ends of the stroke of the piston inside the hydraulic actuator.
In accordance with the foregoing system, each pin-actuated limit switch is encased in a respective sealed canister mounted to the cylinder of the actuator at respective axial positions. For each limit switch, a plunger is deflected by a cam which is rigidly fixed relative to the piston. One cam extends in front of the piston and leads the piston during its forward stroke. The other cam extends to the rear of the piston and leads the piston during its return stroke. The first limit switch is triggered, at the end of the forward stroke of the piston, when its plunger is deflected by the front cam; the second limit switch is triggered, at the end of the return stroke of the piston, when its plunger is deflected by the rear cam.
The aforementioned limit switches operate electrically. Although housed in a sealed canister, the limit switches are prone to failure in the submerged location of the inclined fuel transfer system of a boiling water reactor. Switch failure is due to the intrusion of water vapor and water into the canister. Thus, there is a need for a position detector which will operate reliably even when exposed to water vapor and water.